1. Field of the Invention
This invention relates to compositions for use in the preparation of shell cores and molds, and more particularly, to the preparation of sand precoated with resinous binding agent, particularly adapted for use in the shell process.
2. The Prior Art
Shell cores and molds are used in the casting of metals, and the processes for making them are well known. As an example, the molds are formed from mated sections composed of thin "shells" of sand bonded with a thermoset resin product. The shells generally have a wall thickness of about 0.1 to about 0.5 or more inches.
The individual shell sections are prepared using a mixture of sand and a phenol-formaldehyde novalak resin. These shell molding compositions are referred to in the art as "coated sands" since the mixture of sand and phenol-formaldehyde novalak resin is present in the form of a coating on the individual grains of sand.
Phenol-formaldehyde novalak resins are fusible thermoplastic resins which are prepared by the condensation reaction of phenol with a less than stoichiometric amount of formaldehyde. The resin is converted into an insoluble, cross-linked resinous product upon the addition to the resin, of a methylene generating agent in an amount sufficient to compensate for the stoichiometric deficiency of formaldehyde originally used in the condensation reaction to prepare the resin.
In preparing shell mold sections in accordance with the shell mold process, a large excess of resin coated sand is brought into contact with a metal pattern preheated to a temperature in the range of from 400.degree. to 700.degree. F. The resin adjacent the heated pattern softens under the heat and flows around the sand grains to form a lightly adherent shell on the pattern. The time of contact and the temperature of the pattern determines the thickness of the shell with a given resin coated sand. After a shell of the desired thickness has had time to build up or "invest" in the pattern and conform to the contour thereof, the excess non-adherent resin coated sand is removed from the pattern, and the pattern and adherent shell are then baked at an elevated temperature, e.g. 400.degree. to 700.degree. F in order to advance the novalak resin to its final hardness to form a rigid, self-sustaining thermoset article or shell mold conforming to the shape of the metal pattern. Finally, the hardened shell is stripped from the pattern preparatory to assembly and metal casting operations.
The increasing emphasis on atuomted manufacturing techniques has occasioned modernization of foundry facilities and practices whereby many types of sand molds and cores are now made in large quantities on automated machinery. A limiting factor on the rate at which sand molds and cores can be produced with modern automated foundry machinery has been the length of the investment and cure cycles of the binding agents utilized to adhere the sand particles together. Generally, the time for investment to occur as well as the time to finalize the curing of the binding agent in order to achieve the requisite strength of the mold or core is too slow to achieve optimum utilization of foundry equipment. Too slow cycle time characteristics of the binding agents heretofore used detract from efficient foundry operation and therefore the industry is in a continuous search for thermosetting binder resins which have cycle time characteristics substantially more rapid than other resinous binders heretofore known.